At present, a white LED illumination has been focused as the illumination of the next generation. Conventionally, a discharge type fluorescent lamp and an incandescent bulb used as an illumination device involve problems that a harmful substance such as mercury is contained, life span is short, and heat generation is violent. However, in recent years, a high luminance LED emitting light in a region of blue color and near ultraviolet/ultraviolet, which is required for the white LED illumination, has been developed sequentially. Also, study and development have been actively performed on using the white LED illumination as the illumination of the next generation.
At present, two systems of the white LED illumination are proposed. As one of them, a multi chip type system is given, in which three primary colors of high luminance red LED, a high luminance blue LED, a high luminance green LED, are used. As the other of them, one chip system, which has been developed in recent years, is given, in which LED such as a high luminance ultraviolet LED and a high luminance blue LED, and the phosphor excited by the light having an emission spectrum with a peak in the range from ultraviolet to blue color generated from the LED are combined.
When the aforementioned two systems are compared, the one chip type system has a preferable characteristic as the light source for illumination, such that since it is constituted by combining an LED and the phosphor, it can be small-sized, and the light guide for mixing the emission is simplified, and in addition, the drive voltage, the optical output, and the temperature characteristic of each LED are not required to be taken into consideration, thus realizing cost reduction. Further, by using the phosphor having a broad emission spectrum, the white emission spectrum is approximated the spectrum of the sun-light, and the color rendering properties are possibly improved. This contributes to focusing on the one chip type system as the illumination of the next generation, compared with the multi chip type system.
Further two systems are generally considered for the one chip-type white LED illumination in which the high luminance LED and the phosphor are combined. In one of them, the blue LED with high luminance and the phosphor emitting yellow color by being excited by blue light generated from the LED are combined, and white color is obtained by using a complementary relation between the blue emission of the LED and yellow emission of the phosphor. In the other of them, the LED emitting near ultraviolet/ultraviolet light, the phosphor emitting red (R) color, the phosphor emitting green (G) color, and the phosphor emitting blue (B) color by being excited by the near ultraviolet/ultraviolet light generated from the LED are combined, and the white light is obtained by mixing the colors of the lights obtained from the phosphors of R, G, B and other phosphor.
As the white LED illumination combining the high luminance blue LED and the phosphor emitting yellow color excited by the blue light generated from the LED, the white LED combining the high luminance blue LED and a yellow phosphor (Y, Gd)3(Al, Ga)5O12:Ce (expressed as YAG:Ce) is proposed. Such a white LED has an advantage that by using the complementary relation between the blue light and the yellow light, the kind of the phosphor to be used may be reduced, compared to the system in which near ultraviolet/ultraviolet LED is used. Further, the yellow phosphor YAG:Ce to be used has an excitation spectrum with a peak near the wavelength of 460 nm of emission from the blue LED, thereby emitting light with high efficiency, and the white LED of high luminance can thereby be obtained.
In the latter one chip-type white LED, white color is obtained by mixing the colors of the emission from the phosphors such as R, G, B and so forth, by combining the LED emitting the near ultraviolet/ultraviolet light, the each phosphor emitting red (R), green (G), blue (B) colors excited by the near ultraviolet/ultraviolet light generated from the LED. A method of obtaining the white emission by mixing the emission such as the R, G, B is characterized in that an arbitrary emission color in addition to the white light can be obtained by controlling a combination and a mixing ratio of the R, G, B and also the white emission with excellent color rendering properties is obtained by the relation in a mixed state of colors not using the complementary relation but using the R, G, B. Then, as the phosphor used for such an application, examples are given such as Y2O2S:Eu, La2O2S:Eu, 3.5MgO.0.5MgF2.GeO2:Mn, (La, Mn, Sm)2O2S.Ga2O3:Eu for the red phosphor, ZnS:Cu,Al, SrAl2O4:Eu, BAM:Eu,Mn, Ba2SiO4:Eu for the green phosphor, and BAM:Eu, Sr5(PO4)3Cl:Eu, ZnS:Ag, (Sr, Ca, Ba, Mg)10(PO4)6Cl2:Eu for the blue phosphor.
However, in the former white LED illumination formed by combining the high luminance blue LED and the yellow phosphor (YAG:Ce), the light emission in the longer wavelength side of a visible light region is insufficient, resulting in a bluish white light emission. Then, a red emission spectrum in the vicinity of the wavelength range from 600 nm to 700 nm is insufficient, thereby making it impossible to obtain a slightly reddish white light emission like a lamp bulb, to pose a problem of deteriorated color rendering properties. Further, in regards to an excitation range of the yellow phosphor YAG:Ce, although having an excitation band with highest efficiency in the vicinity of 460 nm wavelength, the yellow phosphor YAG:Ce does not have the excitation band with excellent efficiency in a broad range. Therefore, by the variation in emission wavelength due to the variation in the light emitting element during manufacturing the blue LED, the emission wavelength of the blue LED is deviated from an optimal excitation range of the YAG:Ce-based yellow phosphor, resulting in a situation of losing a wavelength balance of blue color and yellow color. When such a situation occurs, there is the problem that the color tone of the white light obtained by synthesizing the blue light and the yellow light is changed.
In addition, In the latter white LED illumination formed by combining the near ultraviolet/ultraviolet LED and phosphors of R, G, B and so forth, an excitation efficiency and an emission efficiency of the red phosphor and so forth is lower compared with the phosphor of other colors in an excitation range of the near ultraviolet/ultraviolet region. Therefore, the combination of the R, G, B and so forth has no other choice but increase the mixing ratio of only the red phosphor. This causes an insufficient mixing ratio of the phosphor such as the green phosphor improving the luminance, and the white color with high luminance can not be obtained. Further, the red phosphor according to the conventional technique has a sharp emission spectrum, thereby involving the problem that the color rendering properties of the white light obtained is unsatisfactory.
In order to solve the aforementioned problem, it is necessary to obtain the phosphor having the emission spectrum with a broad peak in the range from yellow color to red color (580 nm to 680 nm), and having an excellent excitation band in the longer wavelength side in the range from a near ultraviolet/ultraviolet as an excitation light to a visible light (250 nm to 550 nm). Recently, the phosphors are proposed such as an oxynitride glass phosphor capable of obtaining a broad emission peak in the range of yellow color to red color (for example, see patent document 1), a sialon-based phosphor (for example, see patent documents 2 and 3), and the phosphor containing nitrogen such as silicon nitride-based phosphor (for example, see patent documents 4 and 5), The phosphor containing nitrogen as described above has a larger ratio of convalent bonds, compared with the oxide-based phosphor, and therefore has an excellent excitation band even in the light of 400 nm or more wavelength. Therefore, the phosphor containing the nitrogen is focused as a phosphor for a white LED illumination.
The inventors of the present invention also propose the phosphor having the emission spectrum with an excellent excitation band in the wavelength range from the near ultraviolet/ultraviolet to visible light (250 nm to 550 nm) and a broad peak in the range from yellow color to red color (580 nm to 680 nm), and containing nitrogen (Japanese Patent Application No. 2004-55536).    (Patent document 1) Japanese Patent Laid Open No. 2001-214162    (Patent document 2) Japanese Patent Laid Open No. 2003-336059    (Patent document 3) Japanese Patent Laid Open No. 2003-124527    (Patent document 4) Japanese Patent Laid Open No. 2003-515655    (Patent document 5) Japanese Patent Laid Open No. 2003-277746
However, although the phosphor proposed by the inventors of the present invention is improved in the point of having the emission spectrum with a broad peak in the range from yellow color to red color (580 nm to 680 nm) and an excellent excitation band in the longer wavelength side in the range from a near ultraviolet/ultraviolet as an excitation light to a visible light (250 nm to 550 nm), the problem is involved therein such that the emission intensity is not a satisfactory level. Therefore, even when the white LED illumination is manufactured by combining the near ultraviolet/ultraviolet LED and the blue LED, the mixing ratio of only red phosphor must be increased, thereby making the green phosphor or the like insufficient to improve the luminance, and in some cases, a high luminance white color can not be obtained.
Therefore, the inventors of the present invention prepare samples of various phosphors, while pursuing the cause of not obtaining a sufficient emission intensity in the phosphor, and come to an understanding that a melting point of the raw material to be used is high, thereby making it difficult to progress a solid reaction, resulting in a non-uniform reaction.